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Chapter 4: Igneous Rocks...

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CHAPTER 4: IGNEOUS ROCKS Terms to Remember and Define Andesitic: A texture of igneous rocks in which the crystals are too small for individual minerals to be distinguished without the aid of a microscope.

Aphanitic texture

Assimilation: changing the composition of magma by incorporating surrounding host rock

Bowens reaction series: A concept that illustrates the relationships between magma and the minerals crystallizing from it during the formation of igneous rocks.

Crystallization: The formation and growth of a crystalline solid from a liquid or gas.

Crystal settling: During the crystallization of magma, the earlier-formed minerals are denser than the liquid portion and settle to the bottom of the magma chamber.

Extrusive: Igneous activity that occurs at Earth's surface. (Lava flow)

Decompression melting: Melting that occurs as rock ascends due to a drop in confining pressure.

Felsic: A compositional group of igneous rocks indicating the rock is composed almost entirely of lightcolored silicates.

Pyroclastic Texture: (rhyolitic tuff) relating to, consisting of, or denoting fragments of rock erupted and ejected by a volcano (volcanic ash, pumice, glassy, lacking minerals)

Glass: Natural glass produced when molten lava cools too rapidly to permit recrystallization; a solid composed of unordered atoms.

Glassy texture: A term used to describe the texture of certain igneous rocks, such as obsidian, that contain no crystals.

Groundmass: The matrix of smaller crystals within an igneous rock that has porphyritic texture.

Granitic: Common felsic intrusive igneous rock which is granular and phaneritic in texture. Porphyritic texture. Matrix: 20% quartz and up to 65% alkali feldspar 15% biotite/mica

Intermediate: are roughly even mixtures of felsic minerals (mainly plagioclase) and mafic minerals (mainly hornblende, pyroxene, and/or biotite). There is little or no quartz.

Intrusive: Igneous rock that formed below Earth's surface.

Lava: magma that reaches surface of earth

Mafic: A compositional group of igneous rocks indicating that the rock contains substantial dark silicate minerals and calcium-rich plagioclase feldspar.

Magma: A body of molten rock found at depth, including any dissolved gases and crystals.

Magma mixing: The process of altering the composition of a magma through the mixing of material from another magma body.

Melt: liquid magma, no solid crystals

Partial melting: partial melting of crystals/minerals in magma. Igneous rocks melt over a range of temp. If the liquid is squeezed out after some melting, a melt with higher silica content remains. ^Higher melt ^Higher Silica

Phaneritic texture: crystals are large enough to see with the naked eye, without microscope

Magmatic differentiation: generating more than one rock type from single magma

Phenocryst: large crystal embedded in a matrix of finer grained crystals “odd ball”

Plutonic: igneous rocks that form at depth.

Porphyry: igneous rock with porphyritic texture (ex: granite porphyry)

Porphyritic texture: An igneous rock texture characterized by two distinctively different crystal sizes. The larger crystals are called phenocrysts, whereas the matrix of smaller crystals is termed the groundmass.

Texture: The sizes, shapes, and arrangements of mineral grains in an igneous rock

Ultramafic: Olivine, Pyroxene, Ca-Rich Feldspar- First to crystalline from magma. Composition/Rock Types: Peridotite/Komatiite

Vesicular texture: Aphanitic igneous rocks that contain many small cavities called vesicles Ex: Pumice (rapid cooling and rapid depressurization)

Volatiles: Gaseous components of magma dissolved in the melt; will readily vaporize (form a gas) at surface pressures.

Volcanic: Pertaining to the activities, structures, or rock types of a volcano.

Igneous Rocks to Know and Describe in Terms of Texture, Composition (felsic, mafic, etc.) and Mineral Content (if applicable): Rhyolite Texture: Aphanitic Composition: Felsic Min Content: Quartz and Potassium feldspar

Granite Texture: Phaneritic Composition: biotite and/or muscovite mica Min Content: 25% quartz and 65% or more feldspar

Andesite Texture: Composition: dark silicate minerals. Mineral Content: The other dominant mineral is plagioclase feldspar.

Diorite Texture: Plutonic equivalent of Andesite Composition: Intermediate Mineral Content: Plag Felds, Amphibole, Biotite)

Basalt mid-ocean ridges, lowest Silica content Texture: Aphanitic Composition: Mafic Mineral Content: Pyroxene and calcium rich plagioclase feldspar

Gabbro Texture: Phaneritic Composition: Mafic Mineral Content: Olivine, Pyroxene, Calcium Rich Plagioclase

Peridotite Texture: Phaneritic Composition: Ultra Mafic Mineral Content: Olivine + Pyroxene

Pumice Texture: Vesicular Glassy Composition: Glass Mineral Content: None

Obsidian Texture: Glassy Composition: Compact Glass Mineral Content: None

Volcanic tuff Texture: Pyroclastic Composition: Varies Fragment, Glass, Ash Mineral Content: None

Volcanic breccia Texture: Pyroclastic Composition: Varies Fragment, Glass, Ash Mineral Content: None

Concepts to Know: Know how igneous rocks are classified and named.   

1.Texture-Coarse,Fine,Glassy 2.Chemical-Composition 3.Mineral content -Proportion of light mineral to dark minerals.

How does magma differ from lava? Describe a pyroclastic type of eruption. Lava flows are typically finer grained than intrusive rocks (magma) because the extrusive magma cools quickly so the mineral grains do not have time to grow. Magma is the molten material beneath the Earth's crust. When it reaches the surface and comes out of a volcano, magma becomes lava.

Describe the six types of igneous textures (aphanitic, phaneritic, porphyritic, glassy, vesicular, and fragmental/pyroclastic). Under what conditions does each texture form? Aphanitic: The crystals are too small for individual minerals to be distinguished without the aid of a microscope. Phaneritic: The crystals are roughly equal in size and large enough so the individual minerals can be identified without the aid of a microscope. Porphyritic: Characterized by two distinctively different crystal sizes. The large crystals are Phenocryst, whereas the matrix of smaller crystal is termed ground mass Glassy: The texture of certain igneous rocks, such as obsidian, that contain no crystal. Vesicular: A term applied to aphanitic igneous rocks that contain many small cavities called vesicles. Fragmental/Pyroclastic: the consolidation of individual rock fragments that are ejected during a violent volcanic eruption. How is the rate of magma cooling related to the different types of textures? The rate at which magma cools is the most significant factor in determining the crystal size in an igneous rock: small crystals imply rapid cooling and large crystals imply longer periods of cooling. Igneous rock textures from the smallest to the largest textures are: glassy (no crystals) aphanitic (finegrained), phaneritic (coarse-grained), porphyritic (two grain sizes) & pegmatitic (very coarse-grained).

How do porphyritic textures form? porphyritic (two grain sizes) & pegmatitic (very coarse-grained) Rapid then slow cooling. Change in cooling rate.

What are the roles of pressure and heat in generating magma from solid rock? Role of Heat - the change in temperature with depth is known as the geothermal gradient. Friction at subduction zones and hot rising mantle rocks intruding the crust may create a small amount of magma, but bulk of magma is produced from #2 (pressure).

Role of Pressure -decompression melting. Example: Mid-ocean Ridges & Hotspots - As tectonic plates move apart, the confining pressure is reduced and triggers melting of the mantle. Rocks in the lower crust and upper mantle are near their melting points An increase in confining pressure causes an increase in a rock's melting temperature or conversely, reducing the pressure lowers the melting temperature

Describe decompression melting: Melting that occurs as rock ascends due to a drop in confining pressure

What type of plate boundary would you expect decompression melting to occur? What magma composition is produced within this environment? Mid-Ocean Ridges

Describe partial melting: The process by which most igneous rocks melt. Since individual minerals have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees. If the liquid is squeezed out after some melting has occurred, a melt with a higher silica content results.

How does the composition of a melt produced by partial melting compare with the composition of the parent rock? The melt take the compostion of the heated minerals, while the solid parent rocks remains with the solid minerals that did not melt.

How is Bowen's Reaction Series used to predict the order in which different minerals melt? How does the composition of a melt produced by partial melting compare with the composition of the parent rock? The melt take the compostion of the heated minerals, while the solid parent rocks remains with the solid minerals that did not melt.

How is Bowen's Reaction Series used to predict the order in which different minerals melt? Bowen Reaction series can be used to predict the sequence of melting for mineral crystals formed at low temperatures will melt at low temperatures, and mineral crystals formed at high temperatures will melt at high temperatures. However, the minerals in a particular group, felsic or intermediate, do not all melt at once.

What magma composition does partial melting of peridotite produce? BASALT

Describe how magmatic differentiation changes the composition of magma during crystallization. 2 things change at very distinct levels; different rocks are formed from the same magma chambers.

How is Bowen's Reaction Series used to predict the order in which minerals crystallize as temperature decreases?

What are the first minerals to crystallize from a basaltic magma? Olivine and Calcium Rich Plagioclase (Ca-Plag)

What are the last minerals to crystallize at low temperature in Bowen's Reaction Series? Potassium Feldspar (K-Feldspar), Muscovite Mica, Quartz

What is the sequence of rock compositions produced during differentiation (i.e. basaltic - andesitic granitic)? GRANITIC Peridotite -> Basaltic -> Andesitic -> Granitic -> Rhyolitic

7. Describe the different factors that affect magma composition (crystal settling, magma mixing, assimilation, melting of different source rocks, etc.). a. Crystal settling: Crystallization and removal of minerals through differentiation can change the composition of the remaining magma b. Magma mixing: mixing magmas of different compositions (different magma chambers)

c. Assimilation: Composition of host rock assimilated by magma d. Melting of different source rocks: composition of the original rock(s) melted to form the magma e. Temperature, Pressure, Water content, and mineral composition

8. What is the role of volatiles in generating basaltic magma within subduction zones? -

Causes temp change (water lowers the melting point) Creates melt: at subduction zone, the oceanic plate descends into the mantle

9. Why are rocks of intermediate (andesitic) and felsic (granitic) compositions generally not found in ocean basins? Because they are formed from partial melting of the earths crust

Along which of the three types of plate boundaries are andesitic magmas generated? Convergent Plate Boundaries

Describe two ways in which andesitic magma forms. -Magmatic differentiation of basaltic magma -Basaltic magma migrating upward through continental lithosphere becomes andesitic by assimilating more Si-Rich components of earth’s crust. Describe two ways in which granitic magma forms. -End product of crystallization of andesitic magma -Combination of crystallization and assimilation of continental crust -Direct partial melting of continental crust Where are most granitic rocks found? Mountain Building

TRUE/FALSE

FALSE: Olivine and quartz commonly crystallize together from mafic or basaltic magmas...